In Device-to-Device (D2D) communication, the source and the target are wireless devices, e.g., User Equipments (UEs). Some of the potential advantages are off-loading of the cellular network, faster communication, increased awareness of surrounding wireless devices of interest (e.g., running the same application), higher-quality links due to a shorter distance, etc. Some appealing applications of D2D communications are video streaming, online gaming, media downloading, Peer-to-Peer (P2P), file sharing, etc.
Traditional communication in terrestrial radio networks is via links between UEs and base stations (eNB). However, when two UEs are in the vicinity of each other, then direct device to device (D2D) communication could be considered. Such communication may be dependent on synchronization information from either a base station or a different node such as a cluster head (CH) (a UE acting as synchronization source) providing local synchronization information, or a UE enabled to relay synchronization information from a different synchronization source. The synchronization source SS-eNB from eNB (shown with solid lines) and/or the synchronization source SS-CH from a cluster head CH (shown with dot-dash lines) may be used for intra-cell/cluster communication. The relayed synchronization signal is used for inter-cell/cluster communication. An illustration of synchronization sources from different nodes is shown in FIG. 1.
For in-coverage D2D scenarios, the synchronization reference is provided by the base station eNB, and the D2D resource pool is signaled by the base station eNB to indicate the resource used for D2D. For out of coverage D2D scenarios, the synchronization reference is provided by a cluster head CH.
The signal design of D2DSS (device to device synchronization signal) is under discussion in 3GPP. In current working assumption, D2DSS comprises at least a PD2DSS (primary D2DSS) and may also comprise a SD2DSS (secondary D2DSS). Based on the working assumption, PD2DSS and SD2DSS use a Zadoff-Chu sequence and an M sequence, which are similar to the LTE PSS (primary synchronization signal) and SSS (secondary synchronization signal) respectively. Therefore, it may be advantageous to reuse the LTE PSS and SSS format for the D2DSS as much as possible to reuse the existing timing acquisition circuit to the maximum extent.
So far, 3GPP/RAN1 has assumed common D2D synchronization signals (D2DSS) to support discovery and communication. However, communication may require tighter D2DSS periodicity than discovery, and the above assumption may result in unnecessarily large energy consumption for the purpose of transmitting and/or reading D2DSS for UEs that are only interested in discovery.
If the D2DSS transmission periodicity is decoupled for UEs performing discovery and communication, respectively, there can be confusion for receivers which will receive multiple signals all suitable for synchronization and D2DSS-related measurements, creating a potentially unstable and unpredictable synchronization protocol.